In recent years, researchers and engineers have proposed to use, as a power cable, a superconducting cable, which has a higher transmission capacity than a normal-conduction cable. As the superconducting cable, for example, a superconducting cable may have a structure in which three superconducting-cable cores are twisted together and housed in a heat-insulated pipe, as stated in Patent literature 1.
Each of the superconducting-cable cores comprises, from the center in the following order, a former, a superconducting conductor, an insulating layer, an outer conductor layer, and a protecting layer. Usually, both of the superconducting conductor and the outer conductor layer are formed of superconducting wires. The insulating layer is formed by helically lapping insulating paper to secure a desired electric insulation performance. The superconducting conductor, insulating layer, and outer conductor layer each have a uniform thickness along the entire length of the cable core.
The heat-insulated pipe has a dual-pipe structure composed of an inner pipe and an outer pipe, between which a heat-insulating material is placed. The space in the dual pipe is evacuated in a vacuum. An anticorrosion covering is formed at the outside of the heat-insulated pipe. In the foregoing superconducting cable, usually, the space enclosed by the inner pipe and the cable cores becomes a channel for a coolant.
The superconducting-cable core contracts on account of the cooling to an extremely low temperature by the coolant. The contraction exercises a tension on the superconducting wires, damaging the superconducting wires in some cases. Consequently, in a multicore superconducting cable, an allowance for contraction of the cable cores is secured by placing the cores in the heat-insulated pipe under the condition that the twisting of the cores is slackened.
When such a superconducting cable is used to construct a power line over a long distance, the following types of joint are required. One is an intermediate joint that joints the cores of adjacent cables with each other at an intermediate position of the line. Another is a termination joint that joints the cable core with another electric-power apparatus (a normal-conduction conductor or another conducting member) at the end of the line. (Hereinafter an intermediate joint and termination joint are simply referred to as a joint). In these joints, the end portion of the superconducting cable is stripped off in the shape of a step to expose the superconducting conductor. Next, the exposed superconducting conductor is connected with another conducting member that is also exposed in the same manner as that for the foregoing superconducting conductor. Subsequently, a complementary insulating structure is formed so as to cover the outer circumference of the exposed conductor and the vicinity of it. Usually, both end portions of the complementary insulating structure are each provided with a stress-relief-cone portion that reduces its diameter as the position moves toward the end to relieve the concentration of the electric field at the joint.
Patent literature 1: the published Japanese patent application Tokukai 2006-59695 (see FIG. 5).